1. Field of the Invention
The present invention relates to a clutch operation-assisting device for a straddle type vehicle that reduces the operation force of a friction clutch when the torque transmission is disengaged by the friction clutch.
2. Description of Related Art
A power unit for a motorcycle that includes a friction clutch for engaging and disengaging torque from an engine to a transmission is known. The friction clutch has a clutch shaft, and a friction plate and a clutch plate that contact each other in the shaft direction of the clutch shaft (clutch shaft direction). The friction clutch also includes a pressure plate that engages the friction plate and the clutch plate, and a clutch spring that urges the pressure plate. Under normal conditions, the friction clutch is maintained in a clutch-in state that allows torque transmission by the clutch spring.
The friction clutch has a clutch release mechanism that releases pressure on the pressure plate caused by the clutch spring. The clutch release mechanism is connected to a clutch lever through a clutch wire. Therefore, when a rider grasps the clutch lever, the pressure plate slides against the urging force of the clutch spring, and torque transmission from the friction plate to the clutch plate is disengaged.
In a friction clutch used in a high-powered and high-speed type engine, it is desirable to set an attachment load of the clutch spring high in order to increase torque capacity. However, since the clutch lever operation is performed by human hand, if the attachment load of the clutch spring is set high, the burden of operating the clutch lever is increased.
As a means to solve this, a clutch operation-assisting device that has an assist mechanism for reducing operation force of the clutch lever in a clutch release mechanism to which the clutch wire is connected has been proposed (for example, refer to JP-A-Hei 7-132872 and JP-A-Sho 55-94028).
The clutch operation-assisting device of JP-A-Hei 7-132872 is disposed at the outside of the power unit. However, there are reasons that the clutch operation-assisting device should be disposed at the inside of the power unit. In JP-A-Sho 55-94028, the clutch operation-assisting device is disposed at the inside of the power unit.
As shown in FIG. 9, the clutch operation-assisting device of JP-A-Sho 55-94028 includes a rotatable push lever 525 connected to a clutch wire 528, and an assisting spring 529 that reduces the operation force of the clutch lever. One end 529a of assisting spring 529 is connected to push lever 525, and the other end of assisting spring 529 is connected to a communication member 531 formed in an arc shape. Push lever 525 is fixed to a push screw 517 located on the same straight line (on a straight line extending in a direction from the face to the reverse side of paper of FIG. 9) as the clutch shaft. Push lever 525 rotates together with push screw 517. On the other hand, communication member 531 is rotatably supported to a pin 530, and rotates about pin 530.
When the clutch lever is in a so-called play state (a state where no load is applied when the clutch lever is pulled), assisting spring 529 locates on a straight line M1 that connects a rotation center CO of push lever 525 and pin 530, as shown in FIG. 9. In other words, the extension direction passes through rotation center CO of push lever 525. Therefore, the urging force of assisting spring 529 does not function to rotate push lever 525.
On the other hand, as indicated by a chain double-dashed line in FIG. 9, when the clutch lever is pulled, push lever 525 is dragged and rotates counterclockwise. As a result, the position of assisting spring 529 changes, and extension direction M2 of assisting spring 529 deviates from rotation center CO of push lever 525. Accordingly, a part of the urging force of assisting spring 529 functions to rotate push lever 525 counterclockwise. Therefore, the urging force of assisting spring 529 functions as an assisting force in pulling the clutch lever, and operation force of the clutch lever is reduced.
In this way, in the clutch operation-assisting device described above, assisting spring 529 changes the position as it transitions from the initial position where the clutch lever is in a play state to the disengagement position where the friction clutch is disengaged. In the initial location, extension direction M1 of assisting spring 529 passes through a shaft center (rotation center CO) of push screw 517. Therefore, assisting spring 529 must be disposed along the radial direction from the shaft center of push screw 517, and it is difficult to sufficiently maintain an installation space for assisting spring 529. Also, it is difficult to sufficiently maintain the extension length of assisting spring 529. Therefore, the power unit must be upsized in order to sufficiently maintain installation space for assisting spring 529.